I am building a wireless link between two points using Wireless-N standard. To determine the best position of the antennas, there is a signal strength indicator on the antennas. The problem is the link strength doesn't always correspond to link throughput.

i gave you the check mark myself - as its a good answer. Biggest thing in wireless is he may want to check more than just tcp - so making sure he does some UDP testing would be wise... those tests will really show you where things are good vs bad.
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Glenn KelleyDec 5 '10 at 6:33

iPerf would do much better than would a file on a webserver.
Few reasons - including mod_throttle, bandwidth etc - coming from the server itself.

Why IPERF?

Jed hit the nail on its head suggesting IPERF - here is why:

Iperf is a tool to measure the bandwidth and the quality of a network link. If you would combine IPERF w/ Jperf you have a great graphical interface to use while playing with your link.

A single file coming from a webserver would not be able to give you a clear picture since there is no way to test the quality of the link. The quality of the link is important not just speed.

Using IPERF - you can test the Latency (response time or RTT) - by uinsg the Ping command.
You can also look at any Datagram loss using an IPERF UPD test - heck... Jitter (the variation of the latency across the link) can be measured as well with an IPerf UDP test.

The quality of a link can be tested as follows:
- Latency (response time or RTT): can be measured with the Ping command.
- Jitter (latency variation): can be measured with an Iperf UDP test.
- Datagram loss: can be measured with an Iperf UDP test.

Iperf will also allow you to test bi-directionally which a webserver will not.

Let us know if you need help using IPERF - just respond w/ the operating system your using on each system on both sides of the link.

One last note - what gear are you using - just the radios or spectrum analyzation tools?

Your WIFI gear may be able to help you a great deal.

Channels:

I am assuming you are using 2.4GHZ yes? Reason I ask is that if you are using some of the higher frequencies (such as 5.xGHZ - which MAC's support) they do a good job at closer distances - give excellent speeds BUT hate things like walls in their way.

If you have a spectrum analyzer (ps most all of Ubiquity gear www.UBNT.com have this built in) will help you a great deal.

For example - you may find that channel 1 vs channel 8 is a better choice.We have some links using 2.4Ghz going over a great distance (15+ miles at some places) - and the only variation on speed is the channel - thus allowing us to break apart a great deal of interference.

versus where the CCQ (link quality) is lower overall but we see more bandwidth and a better db level.

In a small office - warehouse etc - channels will make a huge difference, especially if others are close this can really make a difference.

The point here is simple - Location of the antenna's is not the only factor.
If you would agree that using good equipment can make the world of difference than it is fair to say Using a spectrum analyzer will make a Universe of difference.

Waterfall Chart:

This time-based graph shows the aggregate energy collected since the start of an AirView / spectrum analyzer session, over time for each frequency. The power of the energy in dBm is shown across the frequency span and one row is inserted in this graph every few seconds.

It is important to note that the color of the energy designates its amplitude. The darker colors (Blues and darker shades) mean very low to low energy levels at that frequency bin, whereas increasingly brighter colors (ie: Green, Yellow, Orange, and finally Red) designate increasingly higher energy levels at the specific frequency bin.

The Waterfall View's legend (top-right corner) provides a numerical guide associating the various colors to power levels (dBm). The low end of that legend (left) is always adjusted with the calculated noise floor, and the high end (right) is set to the highest detected power level since the start of the session.

It is a great thing to let this run for a long time to see what is happening on the connection from a radio perspective.

Channel Usage Chart:

This graph is ideal for determining the best channel to setup a WiFi network for optimal performance. In the graph, each 2.4GHz WiFi channel is represented by a bar displaying a percentage representing the relative "crowdedness" of that specific channel.This percentage is calculated by analyzing both the popularity and the strength of RF energy in that channel since the start of a AirView session. You as the installer could use this view to optimize a network installation by avoiding highly used channels and setting up new AP's on channels showing the least usage.

Wafeform Chart:

This graph shows the aggregate energy collected since the start of an AirView Session. The power of the energy in dBm is shown across the frequency span. A Blue color will mean energy at a particular strength and frequency appears in the air with a relatively low occurence, whereas increasingly brighter colors (ie: Green, Yellow, Orange, and finally Red) designate energy appearing at a specific power/frequency with higher occurrence. The spectral view over time will essentially display the steady-state RF energy signature of a given environment. You - as the installer could use this view to optimize a network installation by avoiding highly used channels and setting up new AP's on channels showing the least usage.

Real Time Chart:

This is most likely what you want the most...

This graph displays a traditional Spectrum Analyzer in which energy (in dBm) is shown real-time as a function of frequency. There are three traces in this view: Max Hold, this trace will update and hold maximum power levels across the frequency since the start of an AirView session. Average, this trace shows the running average energy across frequency. Real-time, this trace shows the real-time energy seen by the spectrum analyzer device as a function of frequency.

This tool runs for under $100 online - just do a froogle search for Airview.
Combined with IPERF - you can have a really great network for not much work or hassle.

I was thinking something like this, but assuming it's 100Mbps link then 1Gb file will be done in 10 seconds. There's also disk read time which shouldn't be taken into account.
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Adrian GodongDec 5 '10 at 4:38

@Adrian: So try a 10GB file. Or call wget over and over in a loop. Or hack together an ad hoc server with a few lines of Python to stream pseudo-random data in response to an HTTP GET request. Use your imagination! I don't understand your "disk read time" objection, though.
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Steven MondayDec 5 '10 at 5:02